The present invention relates to data communications equipment, e.g., modems, and, more particularly, to echo-canceling modems.
Typically, in a switched data connection, data communications equipment, e.g., a modem, is coupled to a central office of the public switched telephone network (PSTN) through a "local-loop," e.g., a pair of telephone lines. As in all communications systems, the particular communications channel performs some filtering of a transmitted signal communicated from the modem to the central office. This filtering effect of the communications channel is also known as the "channel response," or "frequency response." Unfortunately, the channel response of the local-loop typically does not provide a flat response but, rather, the gain decreases with increasing frequency. In other words, the higher frequency components of the transmitted signal become more attenuated than the lower frequency components of the transmitted signal. At low data rates this effect is not important because the spectrum of the transmitted signal does not have significant, if any, high-frequency components. However, as data rates increase, this effect begins to distort the transmitted signal. In particular, for high-speed data signals the local-loop channel response causes the spectral energy of the high-speed data signal to drop with increasing frequency. Consequently, the spectrum of the received high-speed data signal at the central-office-end of the local loop is severely attenuated at the band-edge of the communications channel.
As a result, it is known in the art to use a technique known as "transmit pre-emphasis" to improve performance of a high-speed data communications system. In transmit pre-emphasis, before a data signal is transmitted, the data signal is shaped by a filter whose frequency response is exactly opposite to the channel response of the communications channel, e.g., the above-mentioned local-loop. That is, the transmit pre-emphasis filter boosts the high frequency components of the data signal and reduces the low frequency signal components of the data signal, thus leaving total transmit power unchanged for the resulting transmitted signal. As a result, the transmit pre-emphasis filter effectively cancels the channel response to result in a flat spectrum for the transmitted signal upon arrival at the other end of the local loop. As used herein, the term "flat spectrum" is defined to mean that the frequency spectrum of the respective signal is equal to a constant, e.g., 1, over a pre-defined frequency bandwidth, e.g., the measured frequency bandwidth of the respective channel, before significant roll-off occurs. The goal in transmit pre-emphasis is to match the characteristics of the local loop, i.e., so that the central office receives a signal with a flat spectrum. Transmit pre-emphasis techniques include fixed pre-emphasis wherein the transmit filter provides a fixed response, or adaptive pre-emphasis wherein the transmit pre-emphasis filter provides a frequency response that is calculated anew for each data connection. U.S. Pat. No. 5,008,903 issued Apr. 16, 1991 to Betts et al. is illustrative of one type of adaptive transmit pre-emphasis technique.
As noted above, the transmit pre-emphasis technique effectively cancels the channel response to result in the central-office-end of the local loop receiving a data signal with a flat spectrum. However, the equipment in the central office does not use pre-emphasis in transmitting signals from the central office to the modem. Consequently, the modem receives high-speed data signals severely attenuated at the band-edge. In the modem, an equalizer, as known-in-the-art, is used to compensate for intersymbol interference present in the received data signal caused by channel attenuation. This compensation by the equalizer includes amplitude correction of the received data signal.
In an echo-canceling high-speed modem, a receiver section includes, among other elements, an echo canceler, to remove echo, and the above-mentioned equalizer. The echo canceler first processes the received data signal to remove any echo signal from the received data signal. Unfortunately, since the received data signal is attenuated by the channel at the band-edges, the echo canceler does not effectively remove components of the far-echo signal at the band-edge (assuming that any channel noise is smaller than the residual echo). As a result, the equalizer, which next processes the received data signal, then attempts to boost the echo-canceled received data signal at the band-edges to compensate for the channel attenuation. Since the echo-canceled received data signal still includes a residual far-echo signal at the band-edges, the residual far-echo signal is also boosted by the equalizer. This effectively results in the introduction of "colored noise" into the remaining components of the receiver, which limits the performance of the high-speed echo-canceling modem.